1. Field of the Invention
This invention relates to the measurement of sheet resistance and thicknesses of samples, and more particularly to the measurement of sheet resistance and thickness of conductive films and substrates.
2. Background of the Related Art
The testing of samples is performed widely in the manufacture of products. For example, the manufacture of integrated circuits (I.C.'s) begins with blank, unpatterned semiconductor wafers. These wafers undergo a number of sometimes critical testing steps before being processed and formed into the final I.C. form. Typically, sheet resistance and wafer (substrate) thickness are tested in the manufacturing process so that the bulk resistivity of a wafer can be determined.
In addition, films of various materials may be applied to substrates and other samples. For example, conductive films are often applied to semiconductor wafers as part of a process for manufacturing integrated circuit chips. Many integrated circuits have devices with submicron geometries, requiring very uniform film thicknesses. It is therefore desirable to have a means for measuring the thickness of the conductive film to ensure uniform film deposition.
Testing of sheet resistance is often done using a test probe assembly that contacts a film 14 formed on a wafer 15, as shown in FIG. 1. The probe assembly 10 usually includes four linearly arranged probes 12a-d, where the two outer probes 12a and 12d direct a constant current I through the film 14 and the two inner probes 12b and 12c read the voltage drop created across the film by the current I on a meter 16. Alternatively, probes 12a and 12c can direct the current I and probes 12b and 12d can read the voltage drop. The constant current I is generated by a current source 18. Following the voltage measurement, the sheet resistance R.sub.s can be calculated from the relationship: ##EQU1## where V is the voltage measured by the two inner probes and I is the current flowing through the film. This formula assumes that all four probes of the probe assembly 10 are equally spaced apart. The thickness of a conductive film on the substrate can be calculated from the relationship: ##EQU2## where p is the (known) resistivity of the film in ohm-cm, and R.sub.s is the measured sheet resistance.
Four point probe measurement systems have been improved over the basic apparatus described above. In U.S. Pat. No. 3,676,775 by A. Dupnock, a method for measuring the resistivity of an epitaxial semiconductor layer is described in which at least two spaced high conductivity diffused regions are formed in the base wafer prior to deposition of the epitaxial layer, and the four point probe is is located directly over the high conductivity regions. In U.S. Pat. No. 4,989,154 by Yamashita et al., a method and apparatus for measuring resistivity using a four-point probe is described in which a correction coefficient is calculated from shape and position information of the wafer and multiplied by the measured resistance value. The Yamashita et al. apparatus includes a computer controller that receives the four point probe measurements through an analog-to-digital (A/D) converter. In U.S. Pat. No. 4,335,350 by J. Chen, an apparatus utilizing two probes is described, in which a four point probe is engaged with one surface of a wafer, and another probe is engaged with the other surface in order to measure leakage current from the first probe. Finally, in U.S. Pat. No. 4,703,252 by Perloffet al., an apparatus and method for testing the sheet resistance of a wafer is described in which multiple test readings are taken by a four point probe at predetermined test locations on the wafer.
While the prior art four point probe apparatus have been successful in measuring the sheet resistance and film thickness of wafers, they tend to exhibit some undesirable characteristics. For example, the prior art uses constant current sources to provide the current through the film. Current sources are expensive and slow to stabilize, often requiring five seconds or so to stabilize for each measurement. This stabilization delay is undesirable for production environments, where fast measurements are desired.
A further problem with the prior art four-point probe apparatus is the presence of offsets and drifts in the current level of the current source. Inaccurate sheet resistance and film thickness values can result from this problem.
A different problem in the prior art is the presence of hysteresis and magnetic effects that occur when different current levels are tested in the wafer. These effects appear when a large jump in current level occurs and can substantially alter a measurement enough so that its accuracy cannot be relied upon.
Yet another problem occurring in the prior art is measuring voltages to a great degree of precision. This is due to prior art four point probes providing a large voltage magnitude at the inner probes which measure the voltage in the film. The larger the magnitude of voltage, the more difficult it is to measure small fractions of the voltage or small changes in voltage.
Another problem occuring in the prior art pertains to the methods of calculating thicknesses of a wafer or other substrate. Prior art methods usually measure the thickness of a wafer or substrate with a micrometer or other instrument in a separate step from the measurement of other characteristics of the substrate. This can lead to inaccuracies in the calculations of bulk resistivity.
What is needed is an apparatus and method that will quickly test samples for sheet resistance, film thickness, and substrate thickness, allow many measurements of a sample's surface to be taken, and allow more samples to be tested in a given amount of time. What is further needed is an apparatus and method that will eliminate the hysteresis and magnetic effects of taking a measurement with a probe and decrease the voltage magnitude of the measured voltage so as to increase the accuracy and precision of the measurements.